1. Field of the Invention
This invention relates to a lip seal for sealing around a shaft, in particular for sealing a ship's propeller shaft in relation to a fluid medium. Such a seal generally can have a clamping part which can be clamped between a first and second back-up ring integral with the shaft housing, a seal lip for sealing against the shaft, and an intermediate part therebetween. The intermediate part can have two zones, the first zone of which is adjacent to the clamping part and the second zone of which supports the seal lip, whereby the two zones form a folded, elastic joint and are disposed angularly with respect to one another. When the pressure of the medium to be sealed out increases beyond a normal pressure, the angle joint of the intermediate part is deformed toward the shaft and into contact with the second back-up ring.
2. Background Information
Lip seals of this type are disclosed by German Laid Open Patent Appln. Nos. 37 18 411, 37 42 080, 38 17 660 and 39 24 270. In such lip seals, when the pressure of the medium to be sealed out increases, the angle joint is deformed towards the shaft in the manner of a roller diaphragm. Such a deformation results in a reduction of the effective surface area of the second zone of the intermediate part, pressurized by the medium to be sealed out, in the direction of the application force of the seal lip edge. Thus the application force is reduced.
This desirable effect is compromised, or reduced, however, by the fact that the second zone of the intermediate part is shortened, whereby the portion of the contact force caused by the elastic righting moment of the intermediate portion is increased. On account of this force acting in the opposite direction, the above-mentioned depressurization effect is reduced, so that at high pressures of the medium to be sealed out, overheating can occur at the lip edge. Such an overheating can cause leaks or even the complete failure of the seal.
German Laid Open Patent Appln. No. 20 64 542 discloses a lip seal on which a back-up ring integral with the housing has a trough which, at normal pressure of the medium to be sealed out, forms a sickle-shaped gap with the sealing body, and when the pressure of the medium to be sealed out increases, this space is filled by the sealing body. However, the trough of the back-up ring is not located between the fastening point of the sealing body and the angle joint, but between the angle joint and the free, or exposed end of the back-up ring, which backup ring runs approximately parallel to the second zone of the intermediate part. In this configuration, accordingly, the seal lip does not retain its unrestricted ability to follow the oscillations of the shaft.